1. Field of the Invention
This invention relates to an automatic transmission, particularly to an automatic transmission for an automobile which is used together with a torque converter, and more particularly to an automatic transmission mechanism which achieves three forward speeds by the use of two planetary gear units.
2. Description of the Related Art
Generally, an automatic transmission is formed of the so-called Simpson type including two single planetary gear units, a sun gear of both planetary units being commonly used and a carrier of the first planetary gear unit being integrally connected to a ring gear of the second planetary gear unit. In this type of a conventional transmission, the three forward speed are achieved as follows. That is, the driving force is selectively inputted by the ring gear and/or sun gear of the first planetary gear unit, the sun gear and the carrier of the second planetary gear unit are retained by a brake or one-way clutch, and the driving force is outputted from the ring gear of the second planetary gear unit integrally connected to the carrier of the first planetary gear unit.
And, the transmission mechanism includes a first (forward) clutch for connecting an input member to the ring gear of the first planetary gear unit and a second (direct) clutch for connecting the input member to the sun gear. These clutches are disposed parallel with respect to each other in the axial direction and are controlled by a separate hydraulic actuator and return spring, respectively.
A conventional four speed automatic transmission employs the aforementioned three speed automatic transmission including two planetary gear units as a base, and in addition an over drive (O/D) mechanism or under drive (U/D) mechanism attached thereto and comprising a planetary gear unit in order to obtain a gear change stage for the four forward speeds.
In recent years, in view of an ever growing demand for FF (front engine front drive), high horse power, etc. of vehicles for example, the automatic transmission is required to be made much smaller. In addition, the transmitting torque capacity is required to be made larger. However, because the conventional automatic transmission mechanism include clutches disposed parallel with respect to each other in the axial direction and operated by separate means as mentioned above, the the transmission must be long in the axial direction, and is thus unable to meet with the demand for a much smaller transmission. Further, the conventional four speed automatic transmission comprising three planetary gear units provided with the O/D mechanism, etc. can hardly satisfy the above demand.
Under the circumstances, there an automatic transmission mechanism has been proposed which additionally attaches two clutches to the conventional so-called Simpson type planetary gear units for obtaining four speeds in Japanese Patent Laid-open Application No. 59(1984)-183147. That is, the mechanism of the automatic transmission is constituted such that a third clutch is disposed between the sun gear of the first planetary gear unit and the sun gear of the second planetary gear unit which are integrally connected with respect to each other in the conventional Simpson type. At the same time, the sun gear of the first gear unit communicates with the carrier of the second gear unit through a fourth clutch, the third clutch is disengaged to separate the first gear unit from the second gear unit, the forth clutch is connected in order to give an input to the carrier of the second gear unit, and at the same time, the sun gear of the second gear unit is fixed to output an overdrive from the ring gear to obtain a fourth speed in addition to the three speeds.
However, even in the above mentioned improved four speed automatic transmission mechanism, the clutches are disposed parallel with respect each other in the axial direction and separate means for operation thereof are included, as in the case of the Simpson type. In addition, many clutches are required to separate both the gear units. Due to the foregoing, the mechanism must be long in the axial direction and therefore, it is impossible to meet with the aforementiond demand for a much smaller transmission. Furthermore, since the output member is located in the front end portion, the clutches are physically impossible to be put together on the front end portion of the mechanism which, if possible, is a desirable location for easy replacement and alternation, and the three speed automatic transmission mechanism and four speed automatic transmission mechanism must be manufactured separately. Due to the foregoing, common use of parts and of an assembly line are difficult to attain, and small production of many kinds of parts often invite a significant increase of cost, and thus the requirements of recent demands for various kinds of vehicles and wide variations thereof are unable to be met.
In view of the above, attention is drawn to the so-called Ravigneaux type automatic transmission mechanism, wherein two units of carriers are integrally formed and which is expected to be made much smaller.
The Ravigneaux type automatic transmission mechanism 1, as shown in FIG. 10, comprises a single planetary gear unit 2 and a dual planetary gear unit 3. An input member I is connected to a sun gear S.sub.2 of the dual unit 3 through a clutch C.sub.1, and also to a sun gear S.sub.1 of the single unit 2 through a clutch C.sub.2 or one-way clutch F.sub.o. The input member I is further connected to a carrier CR.sub.2 supporting dual pinions P.sub.2, P'.sub.2 and to a first carrier CR.sub.1 which is integrally connected to the second carrier CR.sub.2, through a clutch C.sub.o. An output member O is connected to ring gears R1 and R2 of both the units 2 and 3 which are connected with each other and integrally rotated.
And, the Ravigneaux type automatic transmission, as shown in an operation table of FIG. 11, offers first speed through fourth speed (1st through 4th) and reverse (Rev) according to actuation of the respective clutches C.sub.1, C.sub.2 and C.sub.o, the respective brakes B.sub.1, B.sub.2 and B.sub.3, and the respective one-way clutches F.sub.1, F.sub.2 and F.sub.o. In the figure, .circle. denotes coupling of clutches and actuation of brakes and one-way clutches, .DELTA. denotes actuation only during coast time, and .circleincircle. denotes actuation only when gears are shifted and thereafter sychronous rotation is effected.
However, in the Ravigneaux type automatic transmission mechanism, since engine torque is inputted to sun gears S.sub.2 (S.sub.1), the tangential force acting on the sun gear having a smaller diameter is high and in particular this is significant in the case of the first speed wherein the transmitting torque is large. On the other hand, there are problems in that durability of the gear face of the sun gear S.sub.2 is not satisfactory and also, durability of a brake B.sub.3 and one-way clutch F.sub.2 which often receive the reaction force of the carriers CR.sub.1 and CR.sub.2 is problematical. Because of the foregoing, allowable input torque is limited to a low value, thus, recent time demands for high horse power are unable to be met. Any attempt to make the allowable input torque larger requires an increase in the thickness of a gear width of the sun gear. Furthermore, tolerance of retaining means such as brakes, etc. is required to be made larger which again results in a large size of an automatic transmission, and problems arise in loading on vehicles.